Methods for wet chemistry polishing for improved low viscosity printing in solar cell fabrication

ABSTRACT

A method of fabricating a solar cell is disclosed. The method includes forming a polished surface on a silicon substrate and forming a first flowable matrix in an interdigitated pattern on the polished surface, where the polished surface allows the first flowable matrix to form an interdigitated pattern comprising features of uniform thickness and width. In an embodiment, the method includes forming the silicon substrate using a method such as, but not limited to, of diamond wire or slurry wafering processes. In another embodiment, the method includes forming the polished surface on the silicon substrate using a chemical etchant such as, but not limited to, sulfuric acid (H 2 SO 4 ), acetic acid (CH 3 COOH), nitric acid (HNO 3 ), hydrofluoric acid (HF) or phosphoric acid (H 3 PO 4 ). In still another embodiment, the etchant is an isotropic etchant. In yet another embodiment, the method includes providing a surface of the silicon substrate with at most 500 nanometer peak-to-valley roughness.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally tosolar cell manufacture. More particularly, embodiments of the subjectmatter relate to etching and polishing solar cells and techniques formanufacture.

BACKGROUND

Solar cells are well known devices for converting solar radiation toelectrical energy. They can be fabricated on a semiconductor wafer usingsemiconductor processing technology. A solar cell includes P-type andN-type diffusion regions. Solar radiation impinging on the solar cellcreates electrons and holes that migrate to the diffusion regions,thereby creating voltage differentials between the diffusion regions. Ina backside contact solar cell, both the diffusion regions and the metalcontact fingers coupled to them are on the backside of the solar cell.The contact fingers allow an external electrical circuit to be coupledto and be powered by the solar cell. However, improvements in theprocesses used to fabricate solar cells are still needed.

BRIEF SUMMARY

A method for forming a first flowable matrix on a solar cell isdisclosed. The method includes forming a polished surface on a siliconsubstrate and forming a first flowable matrix in an interdigitatedpattern on the polished surface, the interdigitated pattern comprisingfeatures of uniform thickness and width. In an embodiment, the methodincludes forming the silicon substrate using a method such as, but notlimited to, diamond wire or slurry wafering processes. The method alsoincludes etching a silicon substrate using a chemical etchant such as,but not limited to, sulfuric acid (H₂SO₄), acetic acid (CH₃COOH), nitricacid (HNO₃), hydrofluoric acid (HF) or phosphoric acid (H₃PO₄). In anembodiment, the method includes providing a surface of the siliconsubstrate with at most 500 nanometer peak-to-valley roughness. Inanother embodiment, the method includes providing a first flowablematrix including a material having a viscosity in the range of 1-25centipoise (cP). In still another embodiment, the method includesproviding a first flowable matrix including a material such as, but notlimited to, an etch resist ink, a flowable amorphous silicon and aflowable polysilicon. In yet another embodiment, the method includes afirst flowable matrix including a first dopant material having a firstdopant source. In an embodiment, the method includes heating the siliconsubstrate and the first dopant material to a temperature sufficient tocause the first dopant source to diffuse into the silicon substrate. Instill another embodiment, the method includes forming a first dopedregion subsequent to heating the first flowable matrix and the siliconsubstrate. In yet another embodiment, the method further includesforming a second flowable matrix in an interdigitated pattern on thepolished surface, the interdigitated pattern comprising features ofuniform thickness and width. In an embodiment, the second flowablematrix includes a second dopant material having a second dopant source.In another embodiment, the method includes heating the silicon substrateand the second dopant material to a temperature sufficient to cause thesecond dopant source to diffuse into the silicon substrate. In stillanother embodiment, the method includes forming a second doped regionsubsequent to heating the second dopant material and the siliconsubstrate. In yet another embodiment, the method includes a first andsecond dopant source having a dopant such as, but not limited to, boronor phosphorous. In yet another embodiment, the method also includesproviding a first and second flowable matrix composed of a first andsecond metal paste. In an embodiment, the method includes heating thefirst and second metal paste to form a first metal layer and plating asecond metal layer on the first metal layer, where the first metal layerelectrically couples the second metal layer to the first and seconddoped regions. In another embodiment, the method further includesforming the first flowable matrix using a method such as, but notlimited to, ink-jet printing, screen printing or spin coating. In stillanother embodiment, etching the silicon substrate includes isotropicallyetching the silicon substrate to form the polished surface. In yetanother embodiment, the polished surface is instead a semi-polishedsurface. In an embodiment, a semi-polished surface is a surface of thesilicon substrate with at least 8 microns etched away from the siliconsubstrate. In another embodiment, isotropically etching the siliconsubstrate allows for the first flowable matrix to form an interdigitatedpattern having evenly printed lines of a uniform thickness. In stillanother embodiment, prior to isotropically etching the siliconsubstrate, the method includes etching the silicon substrate using ananisotropic etching process to remove excess silicon and form asmoothened surface on the silicon substrate. In yet another embodiment,the method includes anisotropically etching the silicon substrate withPotassium Hydroxide (KOH). In an embodiment, the etchants discussedabove are mixed with deionized (DI) water.

A method of fabricating a solar cell is disclosed. The method includesproviding a solar cell having a front side which faces the sun duringnormal operation and a back side opposite the front side. The methodalso includes forming a polished surface on a silicon substrate using anetchant, where the polished surface has at most 500 nanometerpeak-to-valley roughness and is formed on the back side of the solarcell. The method includes depositing first and second dopant materials,each material having a viscosity in the range of 1-25 centipoise (cP),in an interdigitated pattern on the polished surface, the first andsecond dopant materials including a first and second dopant source,respectively, where the polished surface allows the first and seconddopant materials to form an interdigitated pattern comprising featuresof uniform thickness and width. In an embodiment, the first and seconddopant materials are deposited by industrial printing methods over thepolished surface. The method includes heating the silicon substrate andthe first and second dopant materials to a temperature sufficient tocause the first and second dopant sources to diffuse into the siliconsubstrate. In an embodiment, heating the silicon substrate and the firstand second dopant materials to a temperature sufficient to cause thefirst and second dopant sources to diffuse into the silicon substrateforms a first and second doped regions. The method also includesdepositing a first dielectric layer over the first and second dopedregions. In an embodiment, the first dielectric layer is composed ofsilicon nitride (SiN) or any material commonly used to formanti-reflective regions for solar cells. The method includes forming aplurality of contact openings within the first dielectric layer. In anembodiment, forming the plurality of contact openings includes using awet-etching technique or laser ablation process. The method furtherincludes forming a first metal layer established through the contactopenings over the first and second doped regions, where the first metallayer includes electrically connected grids to the first and seconddoped regions on the back side and plating a second metal layer on thefirst metal layer, where the first metal layer electrically couples thesecond metal layer to the first and second doped regions. In anembodiment, the method includes forming the silicon substrate using amethod such as, but not limited to, diamond wire or slurry waferingprocesses. In another embodiment, the method includes etching a siliconsubstrate using a chemical etchant such as, but not limited to, sulfuricacid (H₂SO₄), acetic acid (CH₃COOH), nitric acid (HNO₃), hydrofluoricacid (HF) or phosphoric acid (H₃PO₄). In still another embodiment,etching the silicon substrate includes etching the silicon substrateusing an isotropic etchant to form the polished surface. In anotherembodiment, isotropically etching the silicon substrate allows for thefirst dopant material to form an interdigitated pattern having evenlyprinted lines of a uniform thickness. In still another embodiment, themethod includes anisotropically etching the silicon substrate to form asmoothened surface prior to isotropically etching.

Another method of fabricating a solar cell is disclosed. The methodincludes providing a solar cell having a front side which faces the sunduring normal operation and a back side opposite the front side. Themethod also includes forming a polished surface on a silicon substrateusing an isotropic etchant, where the polished surface has at most 500nanometer peak-to-valley roughness and is formed on the back side of thesolar cell. The method includes forming first and second doped regionson the back side of the solar cell. The method includes depositing afirst dielectric layer over the first and second doped regions. In anembodiment, the first dielectric layer is an anti-reflective layer overthe back side of the solar cell. The method includes forming a pluralityof contact openings within the first dielectric layer. The method alsoincludes depositing a first metal paste to at least fill at least onecontact opening established through the dielectric layer formed over thefirst and second doped regions, where the topography of the first andsecond doped regions is conformal with the polished surface of thesilicon substrate. The method includes depositing a second metal pasteto connect more than one contact opening filled with the first metalpaste to form interdigitated patterns over the dielectric layer on theback side, where the second metal paste is deposited by industrialprinting methods over the polished surface such that the topography ofthe second metal paste is conformal with the polished surface of thesilicon substrate. The method includes curing the first and second metalpaste to form a first metal layer including electrically connected gridsto the first and second doped regions underneath the dielectric layer onthe back side. The method also includes plating a second metal layer onthe first metal layer, where the first metal layer electrically couplesthe second metal layer to the first and second doped regions. In anembodiment, the method includes forming the silicon substrate using amethod such as, but not limited to diamond wire and slurry waferingprocesses. In an embodiment, the method includes etching the siliconsubstrate using a chemical etchant such as, but not limited to, sulfuricacid (H₂SO₄), acetic acid (CH₃COOH), nitric acid (HNO₃), hydrofluoricacid (HF) or phosphoric acid (H₃PO₄). In another embodiment, the etchantis an isotropic etchant.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the subject matter may be derived byreferring to the detailed description and claims when considered inconjunction with the following figures, wherein like reference numbersrefer to similar elements throughout the figures.

FIGS. 1-3 are cross-sectional representations of a standard process forforming a first flowable matrix on a solar cell;

FIGS. 4A-4D are plan view representations of the solar cell of FIG. 1 inaccordance with a standard process for forming a first flowable matrixon a solar cell;

FIGS. 5-9 are cross-sectional representations of a method for forming afirst flowable matrix on a solar cell;

FIGS. 10-12 are cross-sectional representations of another method forforming a first flowable matrix on a solar cell;

FIGS. 13A-13D are plan view representations of the solar cell of FIG. 5in accordance with an embodiment of the method for forming a firstflowable matrix on a solar cell of FIGS. 5-12;

FIGS. 14-25 are cross-sectional representations of a solar cell beingfabricated in accordance with the methods for forming a first flowablematrix on a solar cell of FIGS. 5-13;

FIGS. 26 and 27 are flowchart representation of methods for forming afirst flowable matrix on a solar cell; and

FIGS. 28 and 29 are flowchart representations of methods for fabricatinga solar cell in accordance with the methods for forming a first flowablematrix on a solar cell of FIGS. 5-13;

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

Techniques for improving manufacturing processes related to etching,cleaning and polishing of solar cells are beneficial as these are anintrinsic part of the standard solar cell fabrication process. Suchtechniques may improve the printed line thickness and linearity on asolar cell during deposition of a printable medium. These or othersimilar embodiments are described below.

Additionally, numerous specific details are set forth, such as specificprocess flow operations, in order to provide a thorough understanding ofthe method and embodiments thereof. It will be apparent to one skilledin the art that the method and its embodiments may be practiced withoutthese specific details. In other instances, well-known fabricationtechniques, such as lithographic and etch techniques, are not describedin detail in order to not unnecessarily obscure the presented methods.Furthermore, it is to be understood that the various embodiments shownin the figures are illustrative representations and are not necessarilydrawn to scale.

As generally known in the art, methods for fabricating a solar cell caninclude providing a silicon substrate, forming doped regions within thesilicon substrate, forming contact regions on the doped regions andplating a metal layer on the contact regions, where an external load isconnected to positive and negative pads along the metal layer to drawcurrent generated by the solar cell. Thus, forming doped regions on asilicon substrate is an integral part of the solar cell fabricationprocess. Various methods for forming doped regions are known. Standardmethods for forming a flowable matrix on a silicon substrate inpreparation to forming doped regions are depicted in FIGS. 1-4 anddiscussed below.

FIG. 1 illustrates a solar cell used in conjunction with a standardprocess of forming a first flowable matrix on a solar cell. The solarcell 100 can include a front side 102 which faces the sun during normaloperation and a back side 104 opposite the front side 102. The solarcell 100 can also include a silicon substrate 110. The silicon substrate110 can include a non-polished surface 101 on the back side 104 of thesilicon substrate 110. The non-polished surface 101 can have surfacefeatures such as unevenly shaped regions, including elevated regions 112and lowered regions 114. The elevated and lowered regions 112, 114 canalso include a peak 116 at the highest point of both regions and avalley 118 being the lowest point between unevenly shaped regions. Thus,the height 113, 115 respectively, for elevated and lowered regions ingeneral can be measured by a peak-to-valley distance. Also, the heightof surface features can vary and be on the average greater than 500nanometers.

With reference to FIG. 2, a standard process for forming a firstflowable matrix on a solar cell is shown. The standard process caninclude using industrial printing methods such as an ink-jet printerwith a nozzle 128, to deposit a first flowable matrix 120 on thenon-polished surface 101 on the back side 104 of the silicon substrate110. A general requirement for ink-jet printing on a substrate is tomaintain a printed line quality, where printed line quality can bemeasured in terms of its deposited line thickness and printed line widthconsistency on a substrate. Also, the first flowable matrix, or flowablematrix, can have any number of different interpretations, where it isused here to include the following: etch resist ink, a flowableamorphous silicon, a flowable polysilicon and various low-viscosityprintable dopants. The first flowable matrix is not necessarily limitedto the described and can also include other substances and classes ofmaterials as well, such as high-viscosity printable materials and thelike.

FIG. 3 illustrates the non-polished surface of FIG. 1 subsequent to thestandard process for forming a first flowable matrix shown in FIG. 2.The first flowable matrix 120 can have areas of unevenly deposited linethickness 135, 137 as seen in FIG. 3 due to the variations in theheights 113, 115, as shown in FIG. 1 on the non-polished surface 101.The variation between the deposited line thicknesses 135, 137 of thefirst flowable matrix 120 can be significant enough to affect the finalprinted line quality. The consistency of the printed line width can alsobe affected.

With reference to FIGS. 4A-4B, there are shown examples of a plan viewfor vertically and horizontally printed lines formed using the method ofFIG. 2. FIGS. 4A and 4B show the first flowable matrix printed on anon-polished surface 133 of a silicon substrate formed using a slurrywafering process. The first flowable matrix 120 can have inconsistentprinted line width as seen along left and right edges 106, 108 forvertically printed lines shown in FIG. 4A and along the top and bottomedges 107, 109 for horizontally printed lines as shown in FIG. 4B. Thedeviation or bleeding at the edges 106, 108, 107, 109 can be caused bythe unevenness or roughness of the topography of the non-polishedsurface 133, where the first flowable matrix 120 follows the topographyof the non-polished surface 133. FIGS. 4C and 4D show the first flowablematrix printed on a silicon substrate formed using a diamond wirewafering process. It is well known that silicon substrates formed usinga diamond wire wafering process also form groove lines 139 along thenon-polished surface 134 of the silicon substrate. Depending on theprint orientation, the first flowable matrix 120 may flow along groovelines 139 providing inconsistent printed line width. FIG. 4C illustratesan example for a printed line formed along the same orientation as thegrooves lines 139. In FIG. 4C, the printed line width for verticallyprinted lines as seen along the left and right edges 103, 105 is similarto FIG. 4A. In contrast FIG. 4D shows an example for a printed lineformed along the opposite orientation as the grooves lines 139. In FIG.4D the printed line width for horizontally printed lines, along the topand bottom edges 146, 147 is much more inconsistent since the groovelines act as channels where the first flowable matrix 120 may flowalong.

With reference to FIGS. 5-9, there are shown methods for forming a firstflowable matrix on a solar cell. One or more embodiments are directed toovercoming the print limitations discussed above, including etching asilicon substrate to form a polished surface. Details and embodimentsare discussed below.

FIG. 5 illustrates a solar cell used in conjunction with a method offorming a first flowable matrix on a solar cell. The solar cell 200,similar to the solar cell of FIG. 1, includes a front side 202 whichfaces the sun during normal operation and a back side 204 opposite thefront side 202. The solar cell 200 also includes a silicon substrate210, where the silicon substrate 210 includes a non-polished surface 201on the back side 204 of the solar cell 200. In an embodiment, the methodincludes forming the silicon substrate 210 using a slurry waferingprocesses. In another embodiment, the method includes forming thesilicon substrate 210 using a diamond wire wafering process. In stillanother embodiment, both the front and back sides 202, 204 havenon-polished surfaces.

FIG. 6 illustrates a method for forming a first flowable matrix on thesolar cell of FIG. 5. The method includes etching the silicon substrate210 using an anisotropic etching process removing excess silicon 231 toform a smoothened surface 232, where forming the smoothened surface 232includes forming sharp corners 236 along surface features of thesmoothened surface 232. In an embodiment, the method includes etchingwith Potassium Hydroxide (KOH). In another embodiment, the methodincludes forming smoothened surfaces on both the front and back sides202, 204 of the solar cell 200.

With reference to FIG. 7, there is shown the smoothened surface of FIG.6. A general drawback to the smoothened surface 232 formed by theanisotropic etching is the possible accumulation of deposited substances211, 221, such as the first flowable matrix, due to the sharp corners236 along elevated and lowered regions 216, 218 of the smoothenedsurface 232. The possible accumulation 211, 221 can leave exposedregions 238 along side walls. The exposed regions 238 can becomediscontinuities within printed lines and can lead to other defects suchas sites for recombination and current loss.

FIG. 8 illustrates the continued method for forming a first flowablematrix on the solar cell of FIG. 7. The method includes performing anisotropic etching process on the smoothened surface to remove excesssilicon 231 and form a polished surface 233 of FIG. 8. In an embodiment,the etching process can include a chemical etchant such as, but notlimited to, sulfuric acid (H₂SO₄), acetic acid (CH₃COOH), nitric acid(HNO₃), hydrofluoric acid (HF) or phosphoric acid (H₃PO₄). In anotherembodiment, the polished surface 233 is instead a semi-polished surface.In still another embodiment, a semi-polished surface is a surface of thesilicon substrate with at least 8 microns etched away from the siliconsubstrate 210. In yet another embodiment, the polished surface 233 hasat most an average of 500 nanometer peak-to-valley roughness. In anembodiment, the etchants discussed above are mixed with deionized (DI)water. In another embodiment, the method includes forming polishedsurfaces on both the front and back sides 202, 204. In still anotherembodiment, isotropically etching the smoothened surface 232 removes thesharp edges 236 of FIG. 7 formed during anisotropic etching, allowing afirst flowable matrix 220 to flow freely covering the exposed regions238 along side walls as shown in FIG. 9.

With reference to FIG. 9, there is shown the continued method of forminga first flowable matrix on the solar cell of FIG. 8. The polishedsurface 233 allows the first flowable matrix 220 to form uniformdeposited line thickness 235, 237 as compared to the unevenly depositedline thicknesses 135, 137 as shown in FIG. 3. In an embodiment, thefirst flowable matrix 220 is deposited using industrial printing methodsuch as an ink-jet printing, where ink-jet printing includes an ink-jetprinter having a dispensing mechanism with a nozzle 228 to deposit thefirst flowable matrix 220. In another embodiment, the first flowablematrix 220 is a first dopant material having a first dopant source. Instill another embodiment, the first flowable matrix 220 includes amaterial having a viscosity in the range of 1-25 centipoise (cP). In yetanother embodiment, the method includes providing a first flowablematrix 220 including a material such as, but not limited to, an etchresist ink, a flowable amorphous silicon or a flowable polysilicon. Inan embodiment, the method includes forming the first flowable matrix 220over the polished surface 233 on the back side 204 of the solar cell 200using a method such as, but not limited to, screen printing or spincoating.

With reference to FIGS. 10-12, there are shown another method forforming a first flowable matrix on a solar cell. Details and embodimentsare discussed below.

With reference to FIG. 10, there is shown a method for forming a firstflowable matrix on the solar cell of FIG. 5. The method includesisotropically etching the non-polished surface 201 of FIG. 5 andremoving excess silicon 231 on the silicon substrate 210 to form apolished surface 233 as shown in FIG. 10. This method is in contrast tothe method shown in FIGS. 6-9, where a smoothened surface is firstformed prior to forming a polished surface on the silicon substrate 210.The method for polishing includes using an acid etching process to formthe polished surface 233, including a chemical etchant such as, but notlimited to, sulfuric acid (H₂SO₄), acetic acid (CH₃COOH), nitric acid(HNO₃), hydrofluoric acid (HF) or phosphoric acid (H₃PO₄). In anembodiment, the etchant is an isotropic etchant. In another embodiment,the method includes forming polished surfaces on both the front and backside 202, 204 of the solar cell 200. In still another embodiment,polishing the back side 204 provides an increase in solar cellefficiency in the range of 0.02-0.1 percent due to unabsorbed lightreflected of the backside of the solar cell. In yet another embodiment,the polished surface 233 includes surface features, where the surfacefeatures have heights 213, 215 measured by a peak-to-valley distancesimilar to that shown in FIG. 1. In an embodiment, the surface featureheights 213, 215 have at most 500 nanometer peak-to-valley height. Inanother embodiment, the polished surface 233 has at most an average of500 nanometer peak-to-valley roughness. In still another embodiment, thepolished surface 233 is a semi-polished surface. In yet anotherembodiment, a semi-polished surface is a surface of the siliconsubstrate 210 with at least 8 microns etched away from the siliconsubstrate 210. In an embodiment, the etchants discussed above are mixedwith deionized (DI) water. In still another embodiment, achemical-mechanical polishing technique is used to etch the siliconsubstrate 210.

FIG. 11 illustrates the continued method of forming a first flowablematrix on the solar cell of FIG. 10. In an embodiment, the firstflowable matrix 220 is deposited using industrial printing methods suchas an ink-jet printing, where ink-jet printing includes an ink-jetprinter having a dispensing mechanism with a nozzle 228 to deposit thefirst flowable matrix 220. In another embodiment, the first flowablematrix 220 is a first dopant material having a first dopant source. Instill another embodiment, the first flowable matrix 220 includes amaterial having a viscosity in the range of 1-25 centipoise (cP). In yetanother embodiment, the method includes providing a first flowablematrix 220 including a material such as, but not limited to, an etchresist ink, a flowable amorphous silicon or a flowable polysilicon. Inan embodiment, the method includes forming the first flowable matrix 220over the polished surface 233 on the back side 204 of the solar cell 200using a method such as, but not limited to screen printing or spincoating.

With reference to FIG. 12, there is shown the polished surface of FIG.11. Similar to the above, the polished surface 233 allows the firstflowable matrix 220 to form uniform deposited line thickness 235, 237 ascompared to the unevenly deposited lined thicknesses 135, 137 shown inFIG. 3. The consistency of the printed line width is also improved asseen in FIG. 13A-13D.

FIGS. 13A-13D illustrate examples of a plan view for vertically andhorizontally printed lines on the polished surfaces of FIG. 9 and FIG.12. FIGS. 13A and 13B show the first flowable matrix 220 printed on thepolished surface 233 of a silicon substrate, where the silicon substratewas formed using a slurry wafering process. The first flowable matrix220 shows a marked improvement in printed line width from FIGS. 4A and4B with no deviations along left and right edges 206, 208 for verticallyprinted lines as seen in FIG. 13A and along the top and bottom edges207, 209 for horizontally printed lines as seen in FIG. 13B. FIGS. 13Cand 13D show the first flowable matrix 220 printed on the polishedsurface 234 of a silicon substrate, where the silicon substrate isformed using a diamond wire wafering process. As discussed above andshown in FIGS. 4C and 4D, diamond wire wafering processes produce groovelines on silicon substrates, where FIG. 4A-D shows that a first flowablematrix can flow along groove lines providing inconsistent printed linedwidth. FIGS. 13C and 13D show that even with groove lines 239, thepolished surface 234 allows the first flowable matrix 220 to have amarked improvement in printed line width as compared from FIGS. 4C and4D. The improvement is seen in both FIGS. 13C and 13D, where there areno deviations along left and right edges 203, 205 for vertically printedlines as seen in FIG. 13C and along the top and bottom edges 246, 247for horizontally printed lines as seen in FIG. 13D.

With reference to FIGS. 14-25, a method fabricating a solar cell ispresented. Details and embodiments of the method are discussed below.

FIG. 14 illustrates a solar cell used in conjunction with a method forfabricating a solar cell. The solar cell 200, similar to the solar cellof FIG. 5, includes a front side 202 configured to face the sun duringnormal operation and a back side 204 opposite the front side 202. Thesolar cell 200 also includes a silicon substrate 210, where the siliconsubstrate 210 includes a non-polished surface 201 on the back side 204of the solar cell 200. In an embodiment, the method includes forming thesilicon substrate 210 using a slurry wafering processes. In anotherembodiment, the method includes forming the silicon substrate 210 usinga diamond wire wafering process.

With reference to FIG. 15, there is shown a method for fabricating asolar cell. The method includes etching the silicon substrate of thesolar cell of FIG. 14, where any of the processes described from FIGS.6-13 can be used to remove excess silicon 231 and form a polishedsurface 233. In an embodiment, the method includes forming polishedsurfaces on both the front and back sides 202, 204 of the solar cell200.

FIGS. 16 and 17 illustrate the continued method for fabricating a solarcell. The method includes forming a first flowable matrix on thepolished surface 233 of the solar cell of FIG. 15. In an embodiment, thefirst flowable matrix is a first dopant material 220, where the firstdopant material 220 includes a first dopant source 225. In anotherembodiment, the first dopant source 225 includes a doping material butis not limited to a positive-type dopant such as boron or anegative-type dopant such as phosphorous. In yet another embodiment, thefirst dopant material 220 has a viscosity in the range of 1-25centipoise (cP). The method also includes depositing the first dopantmaterial 220 over the polished surface 233 using industrial printingmethods such as an ink-jet printing, where the polished surface 233allows the first dopant material 220 to form uniform deposited linethickness and consistently or evenly printed line widths. In anotherembodiment, ink-jet printing includes an ink-jet printer 212 having aprinthead 242 and plurality of nozzles 244. In still another embodiment,the method includes forming the first dopant material 220 using atechnique such as, but not limited to screen printing or spin coating.FIG. 17 shows the solar cell 200 subsequent to forming the first dopantmaterial 220.

With reference to FIG. 18, there is shown the continued method forfabricating a solar cell. The method includes curing 219 the firstdopant material 220. In an embodiment, the method includes curing thefirst dopant material 220 using any curing technique such as thermalcuring, photo-curing or exposure ultraviolet (UV) light.

FIG. 19 illustrates the continued method for fabricating a solar cell.The method includes heating 229 the silicon substrate 210 and the firstdopant material 220 to a temperature sufficient to cause the firstdopant source 225 to diffuse into the silicon substrate 210, forming afirst doped region 224.

With reference to FIG. 20, there is shown the continued method forfabricating a solar cell. The method includes forming a second flowablematrix on the polished surface 233 of the solar cell of FIG. 19. In anembodiment, the second flowable matrix is a second dopant material 222including a second dopant source 227. In another embodiment, the seconddopant source 227 includes a doping material but is not limited anegative-type dopant such as phosphorous or a positive-type dopant suchas boron. In still another embodiment, the second dopant material 222has a viscosity in the range of 1-25 centipoise (cP). The method alsoincludes depositing the second dopant material 222 over the polishedsurface 233 using industrial printing methods such as an ink-jetprinting, where the polished surface 233 allows the second dopantmaterial 222 to form uniform deposited line thickness and consistentlyor evenly printed line widths. In yet another embodiment, the methodincludes forming the second dopant material 222 using any of thetechniques described above.

With reference to FIG. 21, there is shown the continued method forfabricating a solar cell. The method includes curing 219 the seconddopant material 222. In an embodiment, the method includes curing thesecond dopant material 222 using any curing technique such as thermalcuring, photo-curing or exposure ultraviolet (UV) light.

FIG. 22 illustrates the continued method for fabricating a solar cell.The method includes heating 229 the silicon substrate 210 and the seconddopant material 222 to a temperature sufficient to cause the seconddopant source 227 to diffuse into the silicon substrate 210, forming asecond doped region 226. In an embodiment, the first and second dopedregions 224, 226 each include a dopant source 226, 227 such as, but notlimited to a positive-type dopant such as boron or a negative-typedopant such as phosphorous. In another embodiment, the first and seconddoped regions 224, 226 are instead first and second doped polysiliconregions deposited over the silicon substrate 210, where a trench regionseparates the first and second doped polysilicon regions. In stillanother embodiment, the trench region is texturized. In yet anotherembodiment, the method includes providing a dielectric layer between thefirst and second doped polysilicon regions and the silicon substrate210. In an embodiment, the dielectric layer is an oxide layer. Inanother embodiment, the first and second doped polysilicon regions eachinclude a doping material such as, but not limited to, a positive-typedopant such as boron or a negative-type dopant such as phosphorous.

With reference to FIG. 23, there is shown the continued method forfabricating a solar cell. The method includes forming a texturizedregion 267 on the front side 202 of the solar cell 200. The texturizedregion 267 can be one which has a regular or an irregular shapedsurface. The method also includes forming an anti-reflective coating(ARC) 268 over the texturized region 267 on the front side 202 tofurther improve the light absorption properties of the solar cell 200.The method includes forming an anti-reflective coating (BARC) 266 on theback side 204 of the solar cell 200. In an embodiment, the BARC 266 is afirst dielectric layer and the ARC 268 is a second dielectric layer. Inanother embodiment, both the ARC 268 and BARC 266 layers are composed ofsilicon nitride (SiN) or any other material that is commonly used foranti-reflective coating of a solar cell. In still another embodiment,the texturized region 267 is formed using a standard etching process.

FIG. 24 illustrates the continued method for fabricating a solar cell.The method includes forming contact openings 269 through the BARC 266and the first and second dopant material 220, 222 using any lithographicprocesses, including but not limited to wet chemical etching and laserablation. In an embodiment, the contact openings include a plurality ofcontact openings. In another embodiment, the BARC 266 is a firstdielectric layer.

With reference to FIG. 25, there is shown the continued method forfabricating a solar cell. The method includes depositing a first metalpaste 274 to at least fill one contact opening 269 from FIG. 24established through the BARC 266 or a first dielectric layer formed overthe first and second doped regions 224, 226, where the topography of thefirst and second doped regions 224, 226 is conformal with the polishedsurface 233, from FIGS. 15-18, of the silicon substrate 210. The methodalso includes depositing a second metal paste 276 to connect more thanone contact opening filled with the first metal paste 274 to forminterdigitated patterns over the BARC 266 or first dielectric layer onthe back side 204. In an embodiment, the first and second metal paste274, 276 are deposited by industrial printing methods over the polishedsurface 233. In another embodiment, the second metal paste 276 isconformal with the polished surface 233, from FIGS. 15-18, of thesilicon substrate 210. In still another embodiment, the first and secondmetal paste 274, 276 are deposited through a screen printing process. Inyet another embodiment, the first and second metal paste includes analuminum paste. The method includes curing the first and second metalpaste 274, 276 to form a first metal layer. In an embodiment, the firstmetal layer includes electrically connected grids to the first andsecond doped regions 224, 226 underneath the BARC 266 or firstdielectric layer on the back side 204. In yet another embodiment, thefirst metal layer is composed of a layer of aluminum. In an embodiment,the curing process is a heating process. In another embodiment, thefirst metal layer can instead be formed through a standard physicalvapor deposition process, such as sputtering, and a subsequent annealingprocess. The method also includes plating a first and subsequently asecond metal on the first metal layer to form a second metal layer overthe first metal layer. In an embodiment, the second metal layer isformed on the first metal layer using a conventional plating process. Inan embodiment, the first metal layer electrically couples the secondmetal layer to the first and second doped regions 224, 226. In stillanother embodiment, plating a first and second metal includes plating ametal such as, but not limited to, copper, tin, aluminum, silver, gold,chromium, iron, nickel, zinc, ruthenium, palladium, or platinum. Instill another embodiment, the methods described above are used fordifferent types of solar cells such as, but not limited to, aback-contact solar cell, a front-contact solar cell, a monocrystallinesilicon solar cell, a polycrystalline silicon solar cell and anamorphous silicon solar cell.

FIG. 26 illustrates a flowchart of an embodiment for forming a firstflowable matrix on the solar cell as shown in FIGS. 5-9. As discussedabove, a first operation 301 can include providing a solar cell 200having a front side 202 which faces the sun during normal operation, aback side 204 opposite the front side 202 and a silicon substrate 210. Asecond operation 302 can include anisotropically etching the siliconsubstrate 210 to form a smoothened surface 232. A third operation 303can include isotropically etching the smoothened surface 232 to form apolished surface 233 on the silicon substrate 210. The last operation304 can include forming a first flowable matrix 220 in an interdigitatedpattern on the polished surface 233, the interdigitated patterncomprising features of uniform thickness and width.

With reference to FIG. 27, a flowchart of an embodiment for forming afirst flowable matrix on the solar cell of FIGS. 10-12 is shown. Asdiscussed above, a first operation 311 can include providing a solarcell 200 having a front side 202 which faces the sun during normaloperation, a back side 204 opposite the front side 202 and a siliconsubstrate 210. A second operation 312 can include forming a polishedsurface 233 on a silicon substrate 210. The last operation 313 caninclude forming a first flowable matrix 220 in an interdigitated patternon the polished surface 233, the interdigitated pattern comprisingfeatures of uniform thickness and width.

With reference to FIG. 28, there is shown a flowchart of an embodimentfor a method for fabricating a solar cell as illustrated in FIGS. 14-25.As discussed above, a first operation 321 can include providing a solarcell 200 having a front side 202 which faces the sun during normaloperation, a back side 204 opposite the front side 202 and a siliconsubstrate 210. A second operation 322 can include forming a polishedsurface 233 on a silicon substrate 210 using an etchant, where thepolished surface 233 has at most 500 nanometer peak-to-valley roughnessand is formed on the back side 204 of the solar cell 200. A thirdoperation 323 can include depositing first and second dopant materials220, 222, each material having a viscosity in the range of 1-25centipoise (cP), in an interdigitated pattern on the polished surface233, the first and second dopant materials 220, 222 including a firstand second dopant source 225, 227, respectively, where the polishedsurface 233 allows the first and second dopant materials 220, 222 toform an interdigitated pattern comprising features of uniform thicknessand width. A fourth operation 324 can include heating the siliconsubstrate 210 and the first and second dopant materials 220, 222 to atemperature sufficient to cause the first and second dopant sources 225,227 to diffuse into the silicon substrate 210. A fifth operation 325 caninclude depositing a first dielectric layer 266 over the first andsecond dopant regions 220, 222 and first and second doped regions 224,226. A sixth operation 326 can include forming a plurality of contactopenings 269 within the first dielectric layer or BARC 266. A seventhoperation 327 can include forming a first metal layer establishedthrough the contact openings 269 over or above the first and seconddoped regions 224, 226, where the first metal layer includeselectrically connected grids to the first and second doped regions 224,226 on the back side 204. A last operation 328 can include forming asecond metal layer on the first metal layer, where the first metal layerelectrically couples the second metal layer to the first and seconddoped regions 224, 226.

FIG. 29 illustrates a flowchart of an embodiment for a method offabricating the solar cell as shown in FIGS. 14-25. As discussed above,a first operation 331 can include providing a solar cell 200 having afront side 202 which faces the sun during normal operation, a back side204 opposite the front side 202 and a silicon substrate 210. The secondoperation 332 can include forming a polished surface 233 on a siliconsubstrate 210 using an isotropic etchant, where the polished surface 233has at most 500 nanometer peak-to-valley roughness and is formed on theback side 204 of the solar cell 200. A third operation 333 can includeforming first and second doped regions 224, 226 on the back side 204 ofthe solar cell 200. A fourth operation 334 can include depositing afirst dielectric layer or BARC layer 266 over the first and second dopedregions 224, 226. A fifth operation 335 can include forming a pluralityof contact openings 269 within the first dielectric layer or BARC 266. Asixth operation 336 can include depositing a first metal paste 274 to atleast fill at least one contact opening 269 established through thedielectric layer or BARC 266 formed over the first and second dopedregions 224, 226, where the topography of the first and second dopedregions 224, 226 is conformal with the polished surface 233 of thesilicon substrate 210. A seventh operation 337 can include depositing asecond metal paste 276 to connect more than one contact opening 269filled with the first metal paste 274 to form interdigitated patternsover the dielectric layer 266 on the back side 204, where the secondmetal paste 276 is deposited by industrial printing methods over thepolished surface 233 such that the topography of the second metal paste276 is conformal with the polished surface 233 of the silicon substrate210. An eighth operation 338 can include curing the first and secondmetal paste 274, 276 to form a first metal layer including electricallyconnected grids to the first and second doped regions 224, 226 on theback side 204. The last operation 339 can include forming a second metallayer on the first metal layer, where the first metal layer electricallycouples the second metal layer to the first and second doped regions224, 226.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or embodiments described herein are not intended tolimit the scope, applicability, or configuration of the claimed subjectmatter in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the described embodiment or embodiments. It should beunderstood that various changes can be made in the function andarrangement of elements without departing from the scope defined by theclaims, which includes known equivalents and foreseeable equivalents atthe time of filing this patent application.

What is claimed is:
 1. A method of forming a first flowable matrix on asolar cell, the solar cell having a front side which faces the sunduring normal operation and a back side opposite the front side, themethod comprising: forming a polished surface on a silicon substrate,wherein the polished surface comprises a surface of the siliconsubstrate with at most 500 nanometer peak-to-valley roughness; forming afirst flowable matrix in an interdigitated pattern on the polishedsurface, the interdigitated pattern comprising features of uniformthickness and width, wherein the first flowable matrix comprises a firstdopant material having a first dopant source; heating the siliconsubstrate and the first dopant material to a temperature sufficient tocause the first dopant source to diffuse into the silicon substrateforming a first doped region within the silicon substrate; forming asecond flowable matrix comprising a second dopant material having asecond dopant source in an interdigitated pattern on the polishedsurface, the interdigitated pattern comprising features of uniformthickness and width; heating the silicon substrate and the second dopantmaterial to a temperature sufficient to cause the second dopant sourceto diffuse into the silicon substrate forming a second doped regionwithin the silicon substrate; depositing a first dielectric layer overthe first and second doped regions; forming a plurality of contactopenings within the first dielectric layer; forming a first metal layerestablished through the dielectric layer formed over the first andsecond doped regions, wherein the first metal layer compriseselectrically connected grids to the first and second doped regions onthe back side; and forming a second metal layer on the first metallayer, wherein the first metal layer electrically couples the secondmetal layer to the first and second doped regions.
 2. The method ofclaim 1, wherein the silicon substrate is formed using a processselected from the group consisting of a slurry wafering process anddiamond wire wafering process.
 3. The method of claim 1, wherein formingthe polished surface comprises: etching the silicon substrate with ananisotropic etchant to form a smoothened surface; and subsequentlyetching the smoothened surface with an isotropic etchant to form saidpolished surface.
 4. The method of claim 1, wherein forming the polishedsurface comprises etching the silicon substrate with an isotropicetchant.
 5. The method of claim 1, wherein forming the polished surfaceon the silicon substrate comprises etching said silicon substrate usinga chemical etchant selected from the group consisting of sulfuric acid(H₂SO₄), acetic acid (CH₃COOH), nitric acid (HNO₃), hydrofluoric acid(HF) and phosphoric acid (H₃PO₄).
 6. The method of claim 1, wherein thefirst flowable matrix comprises said first and second dopant materialhaving a viscosity in the range of 1-25 centipoise (cP).
 7. The methodof claim 1, wherein the first flowable matrix comprises said first andsecond dopant material selected from the group consisting of a flowableamorphous silicon and a flowable polysilicon.
 8. The method of claim 1,wherein forming the first flowable matrix comprises using said methodselected from the group consisting of ink-jet printing, screen printingand spin coating.
 9. The method of claim 1, wherein the first and seconddopant source comprise said dopant selected from the group consisting ofboron and phosphorous.
 10. A method of forming a first flowable matrixon a solar cell, the solar cell having a front side which faces the sunduring normal operation and a back side opposite the front side, themethod comprising: forming a polished surface on a silicon substrate;forming a first flowable matrix in an interdigitated pattern on thepolished surface, the interdigitated pattern comprising features ofuniform thickness and width, wherein the first flowable matrix comprisesa first dopant material having a first dopant source; heating thesilicon substrate and the first dopant material to a temperaturesufficient to cause the first dopant source to diffuse into the siliconsubstrate forming a first doped region within the silicon substrate;forming a second flowable matrix comprising a second dopant materialhaving a second dopant source in an interdigitated pattern on thepolished surface, the interdigitated pattern comprising features ofuniform thickness and width; heating the silicon substrate and thesecond dopant material to a temperature sufficient to cause the seconddopant source to diffuse into the silicon substrate forming a seconddoped region within the silicon substrate; depositing a first dielectriclayer over the first and second doped regions; forming a plurality ofcontact openings within the first dielectric layer; forming a firstmetal layer established through the dielectric layer formed over thefirst and second doped regions, wherein the first metal layer compriseselectrically connected grids to the first and second doped regions onthe back side; and forming a second metal layer on the first metallayer, wherein the first metal layer electrically couples the secondmetal layer to the first and second doped regions.
 11. The method ofclaim 10, wherein the silicon substrate is formed using a processselected from the group consisting of a slurry wafering process anddiamond wire wafering process.
 12. The method of claim 10, whereinforming the polished surface comprises: etching the silicon substratewith an anisotropic etchant to form a smoothened surface; andsubsequently etching the smoothened surface with an isotropic etchant toform said polished surface.
 13. The method of claim 10, wherein formingthe polished surface comprises etching the silicon substrate with anisotropic etchant.
 14. The method of claim 10, wherein forming thepolished surface on the silicon substrate comprises etching said siliconsubstrate using a chemical etchant selected from the group consisting ofsulfuric acid (H₂SO₄), acetic acid (CH₃COOH), nitric acid (HNO₃),hydrofluoric acid (HF) and phosphoric acid (H₃PO₄).
 15. The method ofclaim 10, wherein the first flowable matrix comprises said first andsecond dopant material having a viscosity in the range of 1-25centipoise (cP).
 16. The method of claim 10, wherein the first flowablematrix comprises said first and second dopant material selected from thegroup consisting of a flowable amorphous silicon and a flowablepolysilicon.
 17. The method of claim 10, wherein forming the firstflowable matrix comprises using said method selected from the groupconsisting of ink-jet printing, screen printing and spin coating. 18.The method of claim 10, wherein the first and second dopant sourcecomprise said dopant selected from the group consisting of boron andphosphorous.